T cells with specific helper function (TM) for the differentiation of thymic cytotoxic T cell precursors (pCTL) into anti-self-TNP effector cells could be enriched in vitro by multiple stimulations with TNP-syngeneic cells after in vivo sensitization with the same hapten. H-2K/D-restricted cytotoxic effector cells (CTL), initially present in such cultures, were often lost after 4 to 6 stimulations without affecting the maintenance of specific TM. I-region homology between responding and hapten-treated stimulating cells was necessary both to induce strong proliferative responses and to maintain the propagation of cells with TH function. The analysis of the interaction between the lines of TH and pCTL indicated that H-2 homology was required between thymocyte responding cells, helper lines and hapten-treated stimulating cells in order to obtain anti-hapten self CTL, but Ig-1 allotype homology was not required.